The present invention relates to an optical transmission system and more particularly to an optical transmission device adapted to an wavelength division multiplexing optical communication and an optical transmission system.
Since a wavelength division multiplexing optical transmission system is capable of realizing transmission of a large amount of data through one optical fiber cable, its demand has expanded rapidly. Further, as the amount of information increases, the number of channels for optical signals allotted to one optical fiber cable is expanded so that a high density is demanded for wavelength intervals between optical signals of adjacent channels.
The wavelength division multiplexing optical transmission system uses a plurality of optical transmission devices generating optical signals at prescribed optical wavelengths and an optical multiplexer for wavelength division multiplexing the optical signals. On the other hand, the wavelength division multiplexed optical signal is, after transmitted through an optical fiber cable, divided by an optical demultiplexer into respective prescribed optical wavelengths and converted in optical receivers to electrical signals.
A conventional optical transmission device used for such an optical transmission system is described in a publication of JP-A-2000-89178, for example.
FIG. 11 shows an arrangement of a conventional optical transmission device, which includes a semiconductor laser 10, an optical modulator 20, a drive circuit 80, a current source 70, a photo detector 55, an optical strength stabilization circuit 71 and an optical strength setting circuit 72. A stimulated emission light is generated by supplying a laser drive current from the current source 70 to the semiconductor laser 10. The stimulated emission light is modulated by the optical modulator 20 to be converted into an optical signal. In this case, the optical strength of the stimulated emission light is monitored by the photo detector 55. In the optical strength stabilization circuit 71, an optical strength setting signal produced by the optical strength setting circuit 72 is compared with an optical strength detection signal produced by the photo detector 55 and the laser drive current is controlled by according to the result of the comparison. As a result, the optical strength of the stimulated emission light is controlled to a constant level and the optical signal is produced at a predetermined optical strength.
In the conventional optical transmission device, the generation/interruption control of the optical signal is conducted by ON/OFF of the laser drive current. In the conventional optical transmission device, the element temperature is changed with variation of the power consumption of the semiconductor laser when the laser drive current is ON/OFF controlled. On the other hand, the semiconductor laser has characteristics such that the optical wavelength of the stimulated emission light varies depending on the element temperature of the semiconductor laser as shown in FIG. 9.
Therefore, when the conventional optical transmission device is applied to the wavelength division multiplexing optical transmission system, the optical wavelength of the optical signal varies in the ON/OFF control of the optical signal so that it is possible that the optical signal is interfered with the optical signal of adjacent optical wavelength channel thereby transmitting wrong optical signal.